Three autosomal trisomies: Down syndrome (trisomy 21), Patau syndrome (trisomy 13), Edward syndrome (trisomy 18); each has an extra copy of the named autosome.
Down syndrome karyotype:
nondisjunction during meiosis gives 47 chromosomes (triplo-21, the common nonfamilial type);
rarely, translocation of chromosome 21 onto another autosome gives 46 chromosomes (familial type).
Trisomy 13 (Patau syndrome) and trisomy 18 (Edward syndrome), clinical features